Pulse width modulation (PWM) is a technology which modulates an electronic signal or a power source in order to transmit information to an electronic device or to control the amount of power sent to an electronic device. PWM occurs in many applications such as power lighting applications for example.
With PWM technology the amount of power sent to a light device such as a Light Emitting Diode (LED), through a switch such as a MOS power transistor or MOSFET (Metal Oxide Semiconductor Field Effect Transistor), can be controlled. The use of PWM technology with low duty cycles requires that the time delay for switching on the MOSFET and the corresponding time delay for switching off the MOSFET are nearly symmetrical. The aim is to ensure that the ON time of the input control signal of the MOSFET equals the ON time of the output voltage of the MOSFET. Thus, the MOSFET can run with low duty cycle. Therefore, the power of the LED is modulated and the lifetime of the LED is greatly enhanced. However, the MOSFET takes generally more time to switch off than to switch on. The unsymmetrical time delays for switching on and off the MOSFET come from the manufacturing process and dynamic characteristics of MOSFET technology.
Indeed the manufacturing process gives rise to mistakes caused by etching and metallization process for example. With these, there is no guarantee that each integrated circuit will have exactly same characteristics for each component thereon. This difference in behaviour of various components of the MOSFET leads further additional time delay between the switching off and switching on processes of a lighting device such as an LED.
US 2003/201811 discloses a method and device for symmetrical slew rate calibration. The aim of US2003/201811 is to directly measure slew rates for push pull driver devices in order to calibrate slew rates. The method uses a driver acting as an oscillator.
U.S. Pat. No. 7,133,790 discloses a method and system of calibrating a control delay time. The method uses a comparison step which uses a predefined pattern.